docs: def safe(self, parentheses=True):
          """
          Returns a string representation with special characters
          replaced by safer characters for use in file names.
          """
          if not self:
              return ""
          string = str(self)
          string = string.replace("**", "_pow_")
          string = string.replace("*", "_mul_")
          string = string.replace("/", "_div_")
          string = string.replace("==", "_eq_")
          string = string.replace("<=", "_leq_")
          string = string.replace(">=", "_geq_")
          string = string.replace("<", "_lt_")
          string = string.replace(">", "_gt_")
          string = string.replace("&&", "_and_")
          string = string.replace("||", "_or_")
          string = string.replace("!", "not_")
          if parentheses:
              string = string.replace("(", "L")
              string = string.replace(")", "R")
          else:
              string = string.replace("(", "")
              string = string.replace(")", "")
          string = string.replace(" ", "")
          return string